Angiogenesis is the process whereby new blood vessels are formed. Angiogenesis, also called neovascularization, occurs normally during embryogenesis and development, and occurs in fully developed organisms during wound healing and placental development. In addition, angiogenesis occurs in various pathological conditions including: ocular diseases such as diabetic retinopathy and macular degeneration due to neovascularization; conditions associated with tissue inflammation such as rheumatoid arthritis and inflammatory bowel disease; and cancer, where blood vessel formation in the growing tumor provides oxygen and nutrients to the tumor cells, as well as providing a route via which tumor cells metastasize throughout the body. Since millions of people around the world are afflicted by these diseases, a considerable effort has been made to understand the mechanisms involved in angiogenesis in order to develop methods for detecting and inhibiting such undesirable angiogenesis.
Angiogenesis occurs in response to stimulation by one or more known growth factors, and also may involve other as yet unidentified factors. Endothelial cells, which are the cells that line mature blood vessels, normally do not proliferate. However, in response to an appropriate stimulus, the endothelial cells become activated and begin to proliferate and migrate into unvascularized tissue to form new blood vessels. In some cases, precursor cells are activated to differentiate into endothelial cells, which form new blood vessels.
Blood vessels are surrounded by an extracellular matrix. In addition to stimulation by growth factors, angiogenesis depends on interaction of the endothelial cells with the extracellular matrix, as well as with each other. The activation of endothelial cells by growth factors and the migration into and interaction with the extracellular matrix and with each other is dependent on cell surface receptors expressed by the endothelial cells. These cell surface receptors, which include growth factor receptors and integrins, interact specifically with particular molecules.
In pathological conditions such as age-related macular degeneration and diabetic retinopathy, decreased availability of oxygen to the retina results in a hypoxic condition that stimulates the secretion of angiogenic growth factors such as vascular endothelial growth factors (VEGF). This secretion induces abnormal migration and proliferation of endothelial cells into tissues of the eye. This results in vascularization of ocular tissues and can induce corneal scarring, retinal detachment and fluid accumulation in the choroid, each of which can adversely affect vision and lead to blindness.
Angiogenesis also is associated with the progression and exacerbation of inflammatory diseases, including psoriasis, rheumatoid arthritis, osteoarthritis, and inflammatory bowel diseases such as ulcerative colitis and Crohn's disease. In inflammatory arthritic disease, for example, influx of lymphocytes into the region surrounding the joints stimulates angiogenesis in the synovial lining. This increased vasculature provides a means for greater influx of leukocytes, which facilitates the destruction of cartilage and bone in the joint. Angiogenic vascularization that occurs in inflammatory bowel disease results in similar effects in the bowel.
The growth of capillaries into atherosclerotic plaques in the coronary arteries represents another pathological condition associated with growth factor induced angiogenesis. Excessive blood flow into neovascularized plaques can result in rupture and hemorrhage of the blood-filled plaques, releasing blood clots that can result in coronary thrombosis.
The involvement of angiogenesis in such diverse diseases as cancer, ocular disease and inflammatory diseases has led to an effort to identify methods for specifically inhibiting angiogenesis as a means to treat these diseases. For cancer patients, such methods of treatment can provide a substantial advantage over currently used methods such as chemotherapy, which kill or impair not only the target tumor cells but also normal proliferating cells in the patient, such as blood cells, epithelial cells, and cells lining the intestinal lumen. Such non-specific killing by chemotherapeutic agents results in side effects that are, at best, unpleasant, and can often result in unacceptable patient morbidity, or mortality. In fact, the undesirable side effects associated with cancer therapies often limit the treatment a patient can receive.